Vascular development and wound healing both depend on dynamic control of endothelial cell adhesion. However, inappropriate loss of adhesion contributes to pathology in inflammatory conditions. Modulation of cell-cell junctions controls endothelial adhesion, and understanding the mechanisms of junction regulation will lend insight into normal vascular development and disease. In this application, we seek to investigate the role of endocytosis in the regulation of vascular endothelial cadherin (VE-cadherin), the main adhesion molecule in endothelial adherens junctions. Despite the importance of VE-cadherin to vessel development, angiogenesis, and inflammation, relatively little is known about the mechanisms of its regulation. One way cells regulate the abundance of plasma membrane proteins such as VE-cadherin is through membrane trafficking. Previous work by our lab demonstrates that p120-catenin (p120), an Armadillo family protein that binds to the VE- cadherin cytoplasmic tail, protects VE-cadherin from rapid internalization and degradation. We hypothesize that disrupting p120 binding to VE-cadherin unmasks an endocytic adaptor binding site, leading to internalization of VE-cadherin and loss of adhesion in endothelial cells. To test this hypothesis, we will pursue the following specific aims. First, we wll identify the amino acid sequences in the VE-cadherin cytoplasmic tail which mediate adaptor binding and endocytosis. We will also test the functional consequences of mutations in VE-cadherin which prevent endocytosis. Second, we will use the ubiquitin ligase K5, which causes rapid down-regulation of VE-cadherin, as a model to determine if disrupting p120 binding is used as a mechanism to control VE-cadherin levels. We will identify the currently unknown mechanism by which K5 causes VE- cadherin down-regulation and its relationship to endogenous pathways for VE-cadherin control. Our long-term goal is to reveal how endothelial cells modulate the strength of cell-cell adhesion, leading to a better understanding of angiogenesis and new treatments for inflammatory disease.